Ceramic packages are used to hold the semiconductor devices and associated circuits used in various electronic apparatus. The ceramic serves as a printed circuit board. In the operation of the semiconductor heat is generated by dissipation of electrical energy in the p-n junctions and, unless the heat is removed, the transistor temperature may rise to destructive levels. The heat must be transferred from the locale of the semiconductor to an external sink, referred to as a "heat sink", so as to prevent a build up of the temperature of the semiconductor, maintaining the semiconductor below the temperature at which destructive thermal runaway occurs.
One known material used as a heat sink is aluminum. The aluminum sheet is connected to the ceramic for the purpose of providing a thermally conductive passage. The large surface area of the aluminum sheet allows the heat to be conducted away to the ambient air or by additional thermally conductive connections to other cooling equipment. In the past the laminates formed of the ceramic board and the aluminum sheet used silicone as the bonding adhesive. Silicone provides an excellent bond between the ceramic and aluminum; one that is strong, is thermally conductive and is capable of meeting requirements of temperature cycling, vibration, humidity and resisted degradation when exposed to solvents, such as isopropyl and acetone, although some silicones degrade after a long term of exposure to MEK, 1,1,1 trichloroethane. However, the silicone adhesive has some disadvantage. Silicone generally requires a primer, such as ECCOSIL Primer S-11, which consists of 25% of polydimethyl Siloxane and 75% of VMP Naphtha to improve adhesion to non-silicone materials such as the ceramic and the aluminum. They require special manufacturing procedures to prevent contamination of the surrounding environment, such as vapor degreasers and surrounding materials such as solder paste; they require long curing times and "rework", the process of disassembling the laminate and then reassembling as might be required to repair a semiconductor within the ceramic package, is difficult and costly. Although substitutes such as adhesives available in the market place were tried, none appeared to be satisfactory with laminates of ten square inches or larger.
An object of the invention is to provide a silicone free substitute for the silicone bonding agent in a ceramic aluminum laminates. A further object of the invention is to provide a ceramic aluminum laminate in which the bonding agent does not contains silicones and possesses good thermal conductivity characteristics. A still further object of the invention is to provide a method of assembling a ceramic printed circuit board to an aluminum heat sink without the use of silicones using ingredients that are available on the market. An additional object of the invention is to provide a ceramic aluminum laminate structure of an area of ten square inches or greater that withstands temperature cycling over a range of -60 degrees centigrade to +125 degrees centigrade for at least 100 cycles, without breaking or bending. It is a still additional object of the invention to provide a thermally conductive adhesive to bond ceramic to aluminum and in which the bond is reworkable.